Cooling arrangement for magnetrons



Feb. 14, 1967 G- A. R. ()JELID COOLING ARRANGEMENT FOR MAGNETRONS 2 Sheets-Sheet 1 Filed April 10, 1964 Q of;

Feb. 14, 1967 G. A. R. @JELID COOLING ARRANGEMENT FOR MAGNETRONS 2 Sheets-Sheet 2 Filed April 10, 1964 INVEN TOR. W

United States Patent Ofilice dfiddfl ld Patented Feb. 14, 867

3,3il4,44) COOLHQG ARRANGEMENT FUR MAGNETRONS GiiranAxel Rune Ojelid, Huskvarna, Sweden, assignor to Husqvarna Vapenfahriks Alitiebolag, a joint-stock company of Sweden Filed Apr. 10, 1964, Ser. No. 358,857 Claims priority, application Sweden, Mar. 4, 60,

- 2,198 6t Claims. ((13. 2191ll.55)

This application is a continuation in part of the U.S.

patent application Serial No. 92,688, issued on April 14,

1964 as Patent No. 3,129,312 to the same inventor as the instant application.

The present invention relates generally to electronic high frequency power tubes, such as magnetrons, and particularly to apparatus for dielectric heating, the electronic equipment of which comprises magnetrons for the heating of food or other substances.

One object of the invention is to provide a magnetron having efficient cooling means which occupy only a very restricted space, particularly in one dimension.

Another object is to reduce the volume required for the electronic equipment of a high frequency oven.

A further object is to provide a high frequency oven having a wall to which its electronic equipment, including a magnetron provided with at least one package of cooling fins and at least one blower therefore, is rigidly secured, said electronic equipment having a very reduced dimension perpendicularly to said wall.

A still further object is to provide a high frequency oven having its electronic equipment, including a magnetron provided with at least one package of cooling fins and at least one blower therefore arranged in close juxtaposition to a wall of the oven, said electronic equipment having a very reduced dimension perpendicularly to said wall.

Nith these and other objects in view the magnetron according to the invention comprises two aligned, spaced apart pole pieces, a substantially U-shaped yoke interconnecting the remote ends of said pole pieces, and a plurality of cooling fins which have a good heat conductivity and are secured, at close intervals, substantially parallel to each other and to the common axis of said pole pieces to the anode block in a close heat exchange relationship therewith.

Further features and advantages of the invention will become apparent from the following description and the drawings which disclose diagrammatically and as nonlimiting examples some preferred embodiments of the invention, and in which:

FIGURE 1 is a perspective view of a high frequency oven and its electronic equipment which comprises a magnetron according to the invention;

FIGURE 2 is a side view of the magnetron as seen from the left in FIGURE 1;

FIGURE 3 is a cross sectional view on line IlIIlI in FIGURE 4 illustrating a further embodiment of a magnetron which could be utilized for heating food or another substance in a high frequency oven according to FIGURE 1; and

FIGURE 4 illustrates the magnetron according to FIGURE 3 as seen from the line IVIV in FIGURE 3.

The apparatus or oven for dielectric heating shown in FIGURE 1 is adapted to be built in and comprises a treatment chamber or oven space in of sheet metal which is united with a front member 24 of sheet metal or wood in which a door (not shown) is arranged in front of the treatment chamber 1a. At one side of the treatment chamber 1a the entire electric equipment of the oven is arranged in the form of a unit which is secured by means of screws 26 to angular brackets 27 or the like on the wall 25 of the treatment chamber la. In front of the unit the front member 24 is perforated or provided, in another way, with openings 23 with a total area of at least cm. for the circulation of air. In the same portion of the front member 24 there is also provided an elongated opening for a bank of setting keys 29 or pushbuttons. These keys 29 are adapted to actuate a multi-polar switch 31 which is rigidly secured to the unit and constitutes its foremost portion. Somewhat behind and laterally of the switch 31 a rectifier assembly 32 is arranged in the unit. This assembly comprises six electrolytic capacitors 33 projecting forwards from an insulating plate 34 and twelve silicium diodes 36 mounted in holders behind the plate. The rectifying assembly 32 constitutes a voltage converter which applies a rectified and amplified voltage to the magnetron 5i and forms a galvanic connection between the electric supply system and the magnetron. The plate 34 is provided with rather big apertures 37 which permit, in conjunction with slots 33 at the sides of the plate, air circulation around the capacitors 33 and the diodes 36. Beside the rectifier assembly 32 a filament current transformer 35 for about 10 va. is mounted. Next behind the rec -r assembly a tangent flow blower 41 with two impellers 42, 2-3, and a driving motor 44 is mounted on a vertical shaft. The impellers have such a shape and rotational direction that is sucked in at the circumferer e of the wheels in a manner known per se and is ischarged backwards through two rectangular openings 4-7. in order to prevent recirculation through the blower l screening plates and 49 are provided inwardly and outwardly respectively of the same. The rearmost part of the unit is constituted by the high-frequency generator 51, viz. a magnetron operating in the frequency range 24G0250O me. The magnetron 51 comprises tvo aligned permanent-ma netic pole-pieces 62, a U-shaped yoke 61 of ferroma netic material the shanks of wl -h are united with the outer ends of the polepieces 62;, and an allelepipcdic anode block 52 located intermediate the pole-pieces 62. The anode block 52 is provided with cooling 53 of sheet copper which soldered into tie anode block 52 of the magnetron and form two cell packages located exactly before the discharge openings 4-7 of the blower. At the output side of th generator 51 a relatively short piece of a wave guide 54 is connected by means of strong fittings (not shown). The wave guide 54 is terminated, at a distance equal to an even multiple of a quarter of the wave length therein, by means of a small flange 56 which permits a capacitive coupling of the treatment chamber in, which also contains a continuation 57 of the wave guide 54. The electromagnetic waves rad .ted into the treatment chamber la are preferably horizontally polarized. Said distance is counted from the wave input, that is the axis of the calotte 53 of the wave guide S i, to the flange 56. There is no galvanic connection at the fiange 55 between the Wave guide 54- and its continuation or to the wall 25 of the chamber 1:: from which the flange 56 is separated by means of a washer 63 of Teflon or the like providing said capacitive coupling.

The entire unit is held together by two rigid Bakelite gables 59 which substantially directly support the individual parts of the unit and besides permit a completely insulated but nevertheless stable attachment of the unit to the above-mentioned angular brackets 27 on the wall 25 of the treatment chamber 1a. With regard to the fact that the magnetron 51 of this device is in galvanic connection with the electric supply system terminals, said insulating construction and said capacitive coupling constitute very important features.

In this embodiment the entire electronic equipment of the oven is encased in a metallic protective casing which is grounded together with the treatment chamber 1a and from which the voltage converter 32 and the magnetron 51 are insulated, galvanically and with respect to alter nating current of the frequency of the electric supply system. It is not necessary that the casing in which the magnetron is located is of metal but it may be made e.g. of plastic. Such a plastic casing could form a supplementary attachment to the Bakelite gables 59.

Within the scope of the invention, the voltage converter could alternatively comprise a transformer isolating the magnetron 51 from the electric supply system, in combination with a rectifier, instead of capacitors 33 and diodes 36. In this instance the protective casing is of metal, and the Teflon washer 63 is omitted, and, furthermore, there is a metallic connection between the flange 56 and the wall 25 of the treatment chamber 1a which is grounded together with the magnetron 51 and the metallic protective casing (not shown).

FIGURE 2 is a fragmentary side view of the magnetron proper on a larger scale. As is evident from this figure the anode block 52 which is made of copper is interposed between the two aligned pole-pieces 62 the outer ends of which are interconnected by means of the U-shaped yoke 61. The anode block 52 is parallelepipedic and has a rectangular cross section. The cooling fins or flanges 53 which are made of copper, aluminum or another heat conducting metal are secured in close heat transferring relationship to the two opposed end faces of a thickened side wall of the anode block 52 and are arranged in two sparse packages laterally with respect to the geometrical axis of the magnetic system of the magnetron and just in front of the exhaust openings 47 of the blower 41. The cooling fins of each package or stack are parallel to each other and are separated by free intervals through which cooling air from the blower may be circulated.

In FIGURE 2 the fins 53 are shown pressed in into individual solts in the respective end faces of the thickened side wall of the anode block 52. In addition to this, they may be soldered thereto. When the anode block as well as the fins are made of copper soldering alone might be sufficient, the slots thus being omitted.

The fins 53 lie in planes which are substantially parallel to the axis of the anode block, as is evident from FIG- URE 2. It is, however, not necessary that the planes of the fins are parallel to the plane of the magnetic system, provided that the blower can be properly located in front of the intervals between the fins 53.

In FIGURES 3 and 4 there is illustrated another embodiment of the air cooling arrangement of the magnetron according to the invention. Here, the same reference numerals as in FIGURES 1 and 2 have been utilized to designate the same or like parts.

As illustrated in FIGURES 3 and 4 the anode block 52 has a cylindrical shape and is provided with a metallic socket 67 for a radio frequency output 64, which projects through an opening 69 in the yoke 61, two heater supporting connections 65 (only one of which is visible in FIG- URE 3) and a connection 66 used for evacuating in conventional manner. The socket 67 is firmly gripped by the springy left hand end of an aluminum socket 68 which is inserted in the opening 69 in the yoke 61 and is divided into a plurality of fingers by axially extending slots 70 (FIGURE 3). The magnetron proper is held in the socket 68 by means of a divided ring 71 the halves of which are clamped together through screws 72 (FIG- URE 4). The yoke 61 is secured to the socket 68 by means of threaded bolts 73, spacing washers 74 being interposed between the yoke 61 and the socket 68 to reduce the transfer of heat from the yoke which becomes heated by the heated cooling air to the magnetron via the socket 68. The socket 68 and accordingly the magnetron proper is secured to the wave guide element 54 by means of an internally threaded ring 76 engaging on the one hand external threads on a flange 77 on the wave guide element 54 and on the other hand a split ring 78 inserted in a circumferential groove in the outside surface of the socket 68.

For its cooling the magnetron is provided with a package of cooling fins 53 which are separated from each other by spacers 80. Each fin consists of a substantially flat rectangle of sheet copper which has a thickness of about 1 mm. and an axial extension which is longer than that of the cylindrical anode block 52, to which it is soldered along the central portion of its inner, longitudial edge. The spacers 80 also consist of substantially flat rectangles of sheet copper which has a thickness of about 2 mms., and they are soldered to the cylindrical wall of the anode block 52 as well as to adjacent cooling fins 53. The package of cooling fins 53 is supplemented with an auxiliary, larger cooling fin 82 which is soldered to the ring 71 and is provided for the cooling of the high frequency output 64. This fin which preferably consists of the same material (copper) as the ring 71 is smoothly rounded to hereby deflect the cooling air around the cylindrical surface of the anode block 52. As is diagrammatically shown by the arrows 81, cooling air from the discharge openings 47 of the blower is forced to flow through the intervals between the fins 53 over and under the top and bottom edges respectively of the spacers 80 and also sweeps the outer lateral faces of the supplementary fin 82 and the outermost fin 53 located on the opposite side of the package. The inner, longitudinal edge of the package of cooling fins 53 and spacers 80 is countersunk or milled with a cutter so that a cylindrical recess is hereby formed therein which has the same curvature as the cylindrical surface of the anode block, to which the package is soldered, as stated above.

As is evident from the above, the fins 53, 82 lie in planes which are substantially parallel to the axis of the anode block, as is evident from FIGURE 3. It is, however, not necessary that the planes of the fins are parallel to the plane of the magnetic system .provided that the blower can be properly located in front of the intervals between the fins 53, since the angular position of the elements 65, 66 may be another than that shown on the drawing.

In FIGURE 3 there is also shown, in dash-dot lines, the wave guide elements 54, 57, the wall 25 of the treatment chamber 1a and a protective casing 83 which may be of plastic and together with the Bakelite gables 59 encloses the magnetron and the voltage converter, and in which outlet openings 84, 85 for the heated cooling air are provided. As is evident from FIGURE 3 the connec tions 65 project through the latter opening.

While the invention has been particularly shown and described with reference to some preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. Thus, the cooling fins may for instance be made integral with the anode block.

What I claim is:

1. In a magnetron of the kind comprising two aligned, spaced apart permanent magnetic pole pieces, a substantially U-shaped yoke interconnecting the remote ends of said pole pieces, and an anode block which is interposed between said aligned pole pieces and is provided with a high frequency output and a pair of heater supporting connections; a cooling arrangement comprising a plurality of cooling fins which have a good heat conductivity and are united, at close intervals, substantially parallel to each other and to the common axis of said pole pieces, with the anode block in close heat exchange relationship therewith, and at least one blower having a discharge opening facing one of the longitudinal edges of each of said cooling fins and the intervals therebetween for blowing cooling air through said intervals.

2. In a magnetron of the kind comprising two aligned, spaced apart permanent magnetic pole pieces, a substanaeoaaoo tially U-shaped yoke interconnecting the remote ends of said pole pieces, and an anode block which is interposed between said aligned pole pieces and is provided with a high frequency output and a pair of heater supporting connections; a cooling arrangement comprising a plurality of substantially fiat copper cooling fins which are soldered at close intervals, substantially parallel to each other and to the common axis of said pole pieces to the anode block, and at least one blower having a discharge opening facing one of the longitudinal edges of each of said cooling fins and the intervals therebetween for blowing cooling air through said intervals.

3. A magnetron according to claim 1, in which one edge of each cooling fin is inserted and clamped in an external groove in a wall of said anode block.

4. In a magnetron of the kind comprising two aligned, spaced apart permanent magnetic pole pieces, a substantially U-sha-ped yoke interconnecting the remote ends of said pole pieces, and a substantially parallelepipedical anode block which is interposed between said aligned pole pieces and has a lateral side wall of substantially greater thickness than its other walls; a cooling arrangement comprising a plurality of cooling fins which have a high heat conductivity and are divided into two packages, the cooling fins of each package being united at close intervals, substantially parallel to each other and to the common axis of said pole pieces, With a respective one of the two opposed end faces of said thickened Wall of said anode block; and a blower having two discharge openings for cooling air, each of said openings facing the intervals between the cooling fins of one of said packages.

5. In a magnetron of the kind comprising two aligned, spaced apart pole pieces, a substantially U-shaped yoke interconnecting the remote ends of said pole pieces, and an anode block which is interposed between said aligned pole pieces and is provided with a high frequency output and a pair of heater supporting connections; a cooling arrangement comprising a plurality of substantially fiat, elongated cooling fins which have a substantially greater extension than the anode block and are separated from each other by spacers coextensive with the anode block, said cooling fins lying in planes which are parallel to the common axis of said pole pieces, said cooling fins and spacers forming a package which is secured to the wall of the anode block in intimate heat exchange relationship therewith; and at least one blower having a discharge opening facing one of the longitudinal edges of each of said cooling fins and the intervals therebetween for blowing cooling air through said intervals.

. In an apparatus for dielectric heating, by means of micro-waves, metal means forming a treatment chamber; a magnetron type micro-wave generator; a voltage converter coupled to said -microwave generator and to a main electric supply system; a transmission line for microwaves coupling said micro-wave generator to said treatment chamber, said micro-wave generator comprising two aligned, spaced apart permanent magnetic pole pieces, a substantially U-shaped yoke interconnecting the remote ends of said pole pieces, an anode block which is interposed between said aligned pole pieces and is provided with a high frequency output, a pair of heater supporting connections and a plurality of substantially fiat sheet-like cooling fins which consist of a metal with great heat conductivity and are secured at close intervals, substantially parallel to each other and to the common axis of said pole pieces, to the anode block in a close heat exchange relationship therewith.

7. In apparatus according to claim 6, at least one blower having a discharge opening facing one of the longitudinal edges of each of said cooling fins and the intervals therebetween for blowing cooling air through said intervals.

8. A magnetron, comprising two aligned, spaced apart pole pieces; a substantially U-shaped yoke interconnecting the remote ends of said pole pieces and having a through hole; and an anode block which is interposed between said aligned pole pieces and is provided with a high frequency output projecting through said hole in the U-shaped yoke, a pair of heater supporting connections, a plurality of substantially fiat, elongated cooling fins which consist of a metal with great heat conductivity and have a substantially greater extension than the anode block, and a corresponding plurality of spacers, substantially co-extensive with said anode block and individually interposed between adjacent cooling fins, said cooling fins and spacers being secured to each other and to the circumferential wall of said anode block in intimate heat exchange relationship therewith.

9. A magnetron, comprising two aligned, spaced apart pole pieces; a substantially U-shaped yoke interconnecting the remote ends of said pole pieces and having a through hole, an anode block which is interposed between said aligned pole pieces and is provided with a high frequency output projecting through said hole in the U-shaped yoke, a pair of heater supporting connections; a plurality of substantially flat, elongated cooling fins which consist of a metal with great heat conductivity and have a substantially greater extension than the anode block; and a corresponding plurality of spacers, substantially co-extensive with said anode block and individually interposed between adjacent cooling fins, said cooling fins and space-rs being secured to each other and to the circumferential wall of said anode block in intimate heat exchange relationship therewith; and a blower having at least one discharge opening which is located in front of the longitudinal edges of the fins and the interspaces therebetween and has a greater extension perpendicularly to the planes of said fins than said package, for directing cooling air against all surfaces of said cooling fins.

it). A magnetron, comprising two aligned, spaced apart pole pieces; a substantially U-shaped yoke interconnecting the remote ends of said pole pieces; a cylindrical anode block, which is interposed between said aligned pole pieces and is provided with an elongated high frequency output and a pair of heater supporting connections; a package of substantially flat, elongated cooling fins, which consist of a metal with great heat conductivity, have a substantially greater extension than the anode block, and are arranged at close intervals in planes, Which are substantially parallel to the high frequency output, said package having a substantially semi-cylindrical recess with the same curvature as the circumferential wall of the cylindrical anode block in one of its walls, those edge portions of the fins defining said recess being secure-d to the circumferential wall of said anode block in intimate heat exchange relationship therewith.

11. in a magnetron of the ldnd comprising two aligned, spaced apart permanent-magnetic pole pieces, a substantially U-shaped yoke interconnecting the remote ends of said pole pieces, and an anode block which is interposed between said aligned pole pieces and is provided with a laterally direct-ed metallic socket for a high frequency output; a cooling arrangement comprising a package of cooling fins which have a good heat conductivity and are united, at close intervals, substantially parallel to each other and to the common axis of said pole pieces, with the anode block in close heat exchange relationship therewith.

12. A magnetron according to claim 11, comprising an additional fin having a main part arranged laterally of said package substantially parallel to the fins thereof, and an end part mounted on said socket in close heat exchange relationship therewith.

13. A magnetron according to claim 12 wherein said cooling fins are parallel to the direction of said socket, and said additional fin is formed with a smooth curved bend between its main part and its end part.

14. In a magnetron of the kind comprising two aligned, spaced apart pole pieces; a substantially U-shaped yoke interconnecting the remote ends of said pole pieces, an anode block which is interposed between said aligned pole pieces and is provided with a laterally directed metallic socket for an elongated high frequency output and a cooling arrangement comprising at least one package of cooling fins which have a good heat conductivity and are united, at close intervals with the anode block in close heat exchange relationship therewith, one of said cooling fins being united with said metallic socket in close heat exchange relationship therewith and at least part of said cooling fins being parallel to each other and to the longitudinal direction of said high frequency output; and at least one blower for blowing cooling air towards said cooling fins and the intervals therebetween.

15. In an apparatus for dielectric heating, by means of micro-waves, metal means forming a treatment chamber; a magnetron ty-pe micro-wave generator; a voltage converter coupled to said micro-wave generator and to a main electric supply system; a transmission line for microwaves coupling said micro-wave generator to said treatment chamber, said micro-wave generator comprising two aligned, spaced apart pole pieces, a substantially U-shaped yoke interconnecting the remote ends of said pole pieces, an anode block which is interposed between said aligned pole pieces and is provided with a laterally directed high frequency output and a cooling arrangement comprising two packages of cooling fins which have a good heat conductivity and are united, at close intervals, substantially parallel to the direction of the high frequency output, with the anode block in close heat exchange relationship therewith; and two blowers, each adapted for blowing cooling air towards the cooling fins and the intervals therebetween of one individual one of said packages.

16. In an apparatus for dielectric heating, by means of micro-waves: metal means forming an oven space; a magnetron type micro-wave generator having two aligned, spaced apart pole pieces, a substantially U-shaped yoke interconnecting the remote ends of said pole pieces and an anode block, which is interposed between said pole pieces and is provided with a laterally directed high frequency output and a cooling arrangement comprising at least one package of cooling fins which have a good heat conductivity and are united, at close intervals substantially parallel to each other and to an adjacent wall of said oven space, with the anode block in close heat exchange relationship therewith; a transmission line for micro-waves coupling said high frequency output to said oven space; a voltage multiplying converter; a protective casing enclosing at least said micro-wave generator and said voltage converter at least one blower for forcing cooling air to sweep said voltage converter as well as the cooling fins of said package of the anode block of said micro-wave generator for the cooling thereof.

17. Apparatus as claimed in claim 15, characterized by the provision of an additional cooling fin which is secured to said high frequency output in close heat exchange relationship therewith and parallel to the direction thereof, so as to be exposed to said cooling air.

18. Apparatus as claimed in claim 16, in which said blower is located between said high frequency generator and said voltage converter and adapted to draw cooling air over one of these members and to blow cooling air over the other one.

19. Apparatus as claimed in claim 16, in which said transmission line comprises a capacitive member constituting a galvanic insulation between said oven space and said micro-wave generator; said voltage multiplying converter comprises capacitors and rectifiers and galvanically interconnects said micro-wave generator with a main electric supply system, and said protective casing is insulated, galvanically and with respect to alternating current of the frequency of the electric supply system, from said micro-Wave generator and from said voltage multiplying converter.

20' Apparatus as claimed in claim 19, in which said protective casing is of metal and is grounded.

21. Apparatus as claimed in claim 19, in which said voltage multiplying converter comprises a transformer and a rectifier, said protective casing is of metal, and there is a metallic connection between said transmission line and said oven space forming metal means, which is grounded.

22. Apparatus as claimed in claim 16, in which said micro-wave generator, said voltage multiplying converter and said blower are located between and secured to two walls of electrically insulating material, confining, together with an adjacent wall of said oven space and an adjacent wall of said protective casing, an air duct ex= tending from the front side of said apparatus towards the rear side thereof.

23. In an apparatus for dielectric heating, by means of micro-waves: metal means forming an oven space; and a microwave generating compartment located adjacent to one wall of said oven space and comprising a magnetron, which has a high frequency output and is provided with at least one package of cooling fins, a voltage converter for the supply of anode voltage, a filament transformer, transmission means connecting the output of the magnetron to said oven space, and a blower for forcing cooling air to sweep the magnetron and its said appurtenant electronic members, said magnetron and appurtenant electronic members being secured in sequence to a removable plate, which together with said wall defines part of an air duct through which said cooling air is forced to flow by said blower.

24. An apparatus as in claim 16 wherein said transmission line comprises a waveguide connected to said anode block by means of a metal socket, said frequency output extending through said socket into said waveguide.

25. An apparatus for dielectric heating by means of microwaves, comprising:

an oven enclosure,

a magnetron having a magnetic means and anode means, said magnetron being located adjacent to a wall of said oven enclosure,

a laterally directed high frequency power output means coupled to said oven enclosure,

cooling flanges located on said anode means and said power output means,

said flanges being perpendicular to a transversal plane of said oven wall,

an air duct defined by said oven wall and a casing for said magnetron, said air duct being substantially perpendicular to said transversal plane, and blower means for generating cooling air towards said magnetron, being connected to said air duct.

References Cited by the Examiner UNITED STATES PATENTS 2,458,802 1/1949 Spencer 31539.53 2,547,503 4/1951 Smith 31539.53 2,716,694 8/1955 Schroeder 219-10.55 2,860,026 11/1958 Long 219-10.55 2,961,520 11/1960 Long 219l0.55 3,076,122 l/1963 Kumpfer 31539.71 3,127,495 3/1964 Polries et al. 21910.55

RICHARD M. WOOD, Primary Examiner. L. H. BENDER, Assistant Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,304,400 February 14, 1967 Goran Axel Runo Ojelid It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, line 7, the claim reference numeral "19'' should read l6 Signed and sealed this 16th day of December 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. E. JR.

Attesting Officer Commissioner of Patents 

1. IN A MAGNETRON OF THE KIND COMPRISING TWO ALIGNED, SPACED APART PERMANENT MAGNETIC POLE PIECES, A SUBSTANTIALLY U-SHAPED YOKE INTERCONNECTING THE REMOTE ENDS OF SAID POLE PIECES, AND AN ANODE BLOCK WHICH IS INTERPOSED BETWEEN SAID ALIGNED POLE PIECES AND IS PROVIDED WITH A HIGH FREQUENCY OUTPUT AND A PAIR OF HEATER SUPPORTING CONNECTIONS; A COOLING ARRANGEMENT COMPRISING A PLURALITY OF COOLING FINS WHICH HAVE A GOOD HEAT CONDUCTIVITY AND ARE UNITED, AT CLOSE INTERVALS, SUBSTANTIALLY PARALLEL TO EACH OTHER AND TO THE COMMON AXIS OF SAID POLE PIECES, WITH THE ANODE BLOCK IN CLOSE HEAT EXCHANGE RELATIONSHIP THEREWITH, AND AT LEAST ONE BLOWER HAVING A DISCHARGE OPENING FACING ONE OF THE LONGITUDINAL EDGES OF EACH OF SAID COOLING FINS AND THE INTERVALS THEREBETWEEN FOR BLOWING COOLING AIR THROUGH SAID INTERVALS. 